Delete expired SMS automatically.

Delete expired SMS whenever we are done processing an SMS-related signal.
In order to minimize additional latency only one SMS is removed at a time.

Change-Id: I56cbe716e52b679c4b94f6cbb4a171306975be2e
Related: OS#2354
diff --git a/include/osmocom/msc/db.h b/include/osmocom/msc/db.h
index 2105d38..a1de7d6 100644
--- a/include/osmocom/msc/db.h
+++ b/include/osmocom/msc/db.h
@@ -51,6 +51,7 @@
 int db_sms_delete_by_msisdn(const char *msisdn);
 int db_sms_delete_sent_message_by_id(unsigned long long sms_id);
 int db_sms_delete_expired_message_by_id(unsigned long long sms_id);
+void db_sms_delete_oldest_expired_message(void);
 
 /* Statistics counter storage */
 struct osmo_counter;